Standard Definition of Displacement. Is there one?
Standard Definition of Displacement. Is there one?
(OP)
I searched and could not find an ASTM, ASME, SAE, or DIN standard defining displacement.
My research lead me back to early "piston pumps" (predating steam engines). There displacement was defined as # of pistons x piston displacement, as these pumps worked on a 1 rev cycle.
I think this method was copied by the otto cycle engines, even though they needed 2 revs to complete a cycle.
When 2 cycle engines appeared, the same "piston pump" approach to displacement was used, but these engines need just one rev to complete all cycles. This advantage was handled in motorcycle racing by allowing about 50-70% more displacement for the 4 stroke bikes, that competed against the 2 stroke bikes.
With the 2 and 4 stroke engines, the "piston pump" rule could always be substantiated by considering each engine as a 1 rev pump to verify displacement.
Enter the Wankel.
The "1.3L" engine found today in Mazdas was very different, with dynamic combustion chambers, and 3 revs to complete a single chambers's 4 strokes. The developers apparently used a different rule to get displacement. They said you have 2 rotors, and each rotor fires once PER REV, a fired chamber is .64L, so displacement is 2 rotors x .64L per rotor = 1.3L displacement ... PER REV.
With this logic, and rotor = piston, a 2 stroke would have the correct displacement (per rev) for 100% efficency. The 4 stroke 5.0L engine would be 2.5L, based on Wankel's "1 rev" method. This is actually a good way to determine displacement, as it is consistent for all engines; 100% VE ingestion per rev.
I don't think anyone is prepared to call their 5.0 V8 a 2.50. So, if one were to apply the "piston pump" rule to the Wankel:
Each rotor has 3 distinct combustion chambers per rotor, that take 3 output shaft revs to fire. With 2 rotors, that's 3.8L fired in 3 revs.
As a wankel pump, each of the sets of 4-otto-cycles of a combustion chamber cycle, provide 2 pumping cycles for a pump (note that the valving and porting are ignored in the basic "piston pump" displacemnt calc). So you take 2 x the fired chambers (pumping strokes) and divide by 3 to get one rev, and you have 2.6L displacement rating, per the "piston pump" method. This is actually the number of chambers that fire in 2 revs, and is an equal basis for rating the extremely common 4 stroke piston engine.
Other than the "piston pump" method, is there any official standard for displacement?
Kevin
My research lead me back to early "piston pumps" (predating steam engines). There displacement was defined as # of pistons x piston displacement, as these pumps worked on a 1 rev cycle.
I think this method was copied by the otto cycle engines, even though they needed 2 revs to complete a cycle.
When 2 cycle engines appeared, the same "piston pump" approach to displacement was used, but these engines need just one rev to complete all cycles. This advantage was handled in motorcycle racing by allowing about 50-70% more displacement for the 4 stroke bikes, that competed against the 2 stroke bikes.
With the 2 and 4 stroke engines, the "piston pump" rule could always be substantiated by considering each engine as a 1 rev pump to verify displacement.
Enter the Wankel.
The "1.3L" engine found today in Mazdas was very different, with dynamic combustion chambers, and 3 revs to complete a single chambers's 4 strokes. The developers apparently used a different rule to get displacement. They said you have 2 rotors, and each rotor fires once PER REV, a fired chamber is .64L, so displacement is 2 rotors x .64L per rotor = 1.3L displacement ... PER REV.
With this logic, and rotor = piston, a 2 stroke would have the correct displacement (per rev) for 100% efficency. The 4 stroke 5.0L engine would be 2.5L, based on Wankel's "1 rev" method. This is actually a good way to determine displacement, as it is consistent for all engines; 100% VE ingestion per rev.
I don't think anyone is prepared to call their 5.0 V8 a 2.50. So, if one were to apply the "piston pump" rule to the Wankel:
Each rotor has 3 distinct combustion chambers per rotor, that take 3 output shaft revs to fire. With 2 rotors, that's 3.8L fired in 3 revs.
As a wankel pump, each of the sets of 4-otto-cycles of a combustion chamber cycle, provide 2 pumping cycles for a pump (note that the valving and porting are ignored in the basic "piston pump" displacemnt calc). So you take 2 x the fired chambers (pumping strokes) and divide by 3 to get one rev, and you have 2.6L displacement rating, per the "piston pump" method. This is actually the number of chambers that fire in 2 revs, and is an equal basis for rating the extremely common 4 stroke piston engine.
Other than the "piston pump" method, is there any official standard for displacement?
Kevin





RE: Standard Definition of Displacement. Is there one?
RE: Standard Definition of Displacement. Is there one?
RE: Standard Definition of Displacement. Is there one?
But you knew that.
The only question is what is the displacement of a rotary. I've always considered it to be 3.9 liters.
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RE: Standard Definition of Displacement. Is there one?
Cheers
Greg Locock
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RE: Standard Definition of Displacement. Is there one?
To be pedantic about it, if you change it for average ingestion per full cycle instead of per induction stroke, how do you account for superchargers?
Regards
Pat
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RE: Standard Definition of Displacement. Is there one?
Super chargers and turbos are beyond the scope of my question, but you'd start with pressure ratio and lots of corrections for shaft work done, and back pressure, and intercooling, etc.
Apparently the answer is no standard for disp't rating.
The Mazda wankel is, assuming 100% VE:
1.3L ... based on air injested in 1 rev.
3.8L ... based on one complete engine cycle of 3 revs.
2.6L ... based on the "piston pump" type rating I used.
For common NA engines, the ancient "piston pump" rating method is correct for all but the 2-stroke, which is a well known error, at least in the motorcycle racing world.
Kevin
RE: Standard Definition of Displacement. Is there one?
Cheers
Greg Locock
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RE: Standard Definition of Displacement. Is there one?
Translation?
RE: Standard Definition of Displacement. Is there one?
""Do not teach your grandmother how to suck eggs""
The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor
RE: Standard Definition of Displacement. Is there one?
In my 'day' or notebook, or whatever you want to say displacement is total swept volume.
Swept volume being the_swept_volume, x amount of cylinders = displacement.
Thats the most standard definition of displacement you can get which is fine for 98% of the time. The other 2% depends.
''Other than the "piston pump" method, is there any official standard for displacement?''
Wiki>
Engineering
''Engine displacement'', the total volume of air/fuel mixture an engine can draw in during one complete engine cycle.
Take what you want from that I guess.
''assuming 100% VE''
Thats in with the 2% I mentioned above.
Brian,
RE: Standard Definition of Displacement. Is there one?
Regards
Pat
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RE: Standard Definition of Displacement. Is there one?
- Steve
RE: Standard Definition of Displacement. Is there one?
"The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them." Old professor [/quote]
Politely disagree. I relied on the principles of Physics and related formulas which were burned in my cpu from school, not memorization, to save millions of dollars for the company I worked for. This quote would say the good engineers were the ones I saw going to the ME PE test with shopping carts full of books, and I was the bad engineer who went to the test with 3 books, left early, and passed with an 87 or 88%. Sorry Brian, no LOL for Greg from me.
RE: Standard Definition of Displacement. Is there one?
RE: Standard Definition of Displacement. Is there one?
Since it takes 3 revs to fire all 6 chambers, it can be visualized as equivelant to a 3.8L 6 cyl engine, with a .667 gear reduction at the original output shaft. This provides the same otto cycle graph for each chamber, vs revs, for the 6 cylinder engine and the wankel.
RE: Standard Definition of Displacement. Is there one?
RE: Standard Definition of Displacement. Is there one?
Pretty soon you are talking shaft hp, and with fuel flow numbers, BSFC, or hp/pound, or in other words...real numbers.
RE: Standard Definition of Displacement. Is there one?
Not me, just looking for a standard for rated displacement, and found none, and unfortunately found one low class remark. I'll stick with the ancient "piston pump" method I unearthed before. All piston engines on the road are correctly sized, except the 2-strokes are still undersized, and the 13B goes from 1.3L to 2.6L.
You don't see existing engines rated displacement, diesel or gas, a function of turbos and/or pulley driven superchargers.
RE: Standard Definition of Displacement. Is there one?
- Steve
RE: Standard Definition of Displacement. Is there one?
RE: Standard Definition of Displacement. Is there one?
Probably because displacement is a fairly useless measurement, other than for "bragging rights" at the local drinking establishment? See Steve's post.
RE: Standard Definition of Displacement. Is there one?
Yeah, what about a Hemi?
RE: Standard Definition of Displacement. Is there one?
Racing bodies put on correction factors to keep competition even.
Apart from that it does not really matter unless trying to do original estimate of fuel flow requirements
Regards
Pat
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RE: Standard Definition of Displacement. Is there one?
The wankel is easily compared to a 4 stroke otto engine, which it is, with rotor faces vs piston tops. It's a clever version that uses "shared" combustion chambers. See my post at 14:06 for a direct piston engine equal to the 13B. To take it a step further, the stroke would only be 1.38" and the piston dia 3.0" .
RE: Standard Definition of Displacement. Is there one?
(6) .65L cylinders ( same as 6 .65L "1/3 moon" shaped pockets in 13B)
and, a 1.5 planetary-gear speed increaser on the output shaft of the 6 cyl engine, presenting the functional output shaft to the flywheel at 1.5X crank speed.
This creates the same duration of the otto cycle strokes.
Kevin
RE: Standard Definition of Displacement. Is there one?
''In auto racing, the displacement of a Wankel engine is usually doubled for classing purposes. This is of course a marketing ploy and wrong. Using only a single face per rotor instead of three results in the nominal displacement being a third of actual. For calculating taxes in Japan, the displacement of Wankel engines is defined as the equivalent of 1.5 times the nominal displacement, so the 1300 cc 13B engines are taxed as 1950 cc, whereas the actual displacement is 3900 cc''
Pulled off the internet to save time.
See Pats last sentence in his last post.
Brian,
RE: Standard Definition of Displacement. Is there one?
This reference was pre-13B, Mazda's version with 17.5mm offset.
RE: Standard Definition of Displacement. Is there one?
Thus, the Mazda RX-7, RX-8 Wankel engines with 2 rotors displace 1.3 l per crankshaft revolution, which is why it's named 13B and not 39B.
1.3 l is the same volume a 2.6 l piston 4-stroke engine would pump (assuming same VE), which is why it is taxed, raced, compared to a 2.6 l conventional engine.
RE: Standard Definition of Displacement. Is there one?
Naturally, the 1.3 l displacement has to be doubled for classing purposes unless the Wankel engine would race against 2-stroke piston engines. If one doesn't believe it - here's an animation showing that the rotor does only 1/3 of the revolution of the crankshaft revolution: http://de.wikipedia.org/wiki/Wankelmotor
Piston as well as Wankel engines are essentially positive displacement pumps, which can easily be compared and classified (unlike centrifugal and axial flow pumps).
RE: Standard Definition of Displacement. Is there one?
Using that logic, basically injested air per rev, a mustang 5.0 should be rebadged a mustang 2.5 ... ain't gonna happen. That was the clever logic that fooled people into thinking the 13B had the highest specific power of all NA automotive engines. The 5.0 is based an equivelant pump, swept volume in one rev.
Globi5, are you speaking of two different volumes, in your 1st and 3rd paragraph?
Kevin
RE: Standard Definition of Displacement. Is there one?
1) I found here that there is no SAE standard for displacement.
2) My research showed the first dicussion of volumetric "displacement" for machines occured with ancient piston pumps, predating the steam engine.
3) Otto cycle engines adopted this "piston pump" method for displacement rating, # pistons x area x stroke, or swept volume. This had a flaw in that it under-rated 2 stroke power potential. It also was independent of the number or revs to complete an engine cycle.
4) When the wankel was developed, they took advantage of the lack of a standard. The swept volume would have put it at 3.8L. So unlike the other otto's they assumed a "per rev" basis for displacement, as 2 chambers were swept per rev.
But, using the old "piston pump" method, you see how much is pumped per rev, as a pump where the 4 strokes for a single otto cycle yield )2) 2-stoke pump cycles. So:
(3.8L x 2) total pumped in 3 revs / 3 = 2.6L pumped per rev.
Kevin
RE: Standard Definition of Displacement. Is there one?
2. For the reason above the rotary engine pumps 3 x 1.3 l = 3.9 l in 3 revolutions and definitely not 2 x 3.9 l and definitely neither 2.6 l per revolution.
3. A valveless 2 stroke piston engine pumps as much volume as it displacement indicates. And so does this (also valveless) rotary engine.
4. A four stroke engine only displaces half of its volume, because it only pumps its displacement every second revolution. (Thanks to its inlet valves which only open every second revolution.)
If you were to badge the displacement of a 4 stroke piston engine to half of its displacement, people would then ask: Why is a 4 stroke piston so much bigger than a 2 stroke or a piston compressor or steam engine, even though displacement is defined as http
5. I don't recall any Mazda official ever trying to fool the public regarding the classification of its rotary engine, because nobody can fool simple geometry and nobody really cares anyway.
6. I don't even care about the displacement definition, I just tried to help regarding your question: "Can anyone check my equivalent displacement 3.9L"
And I also wanted to let you know that this wiki sentence above is false. But if you don't want to know the facts, don't ask for it and put whatever badges on rotary engines you prefer.
RE: Standard Definition of Displacement. Is there one?
I said, configured as a true pump, where the four strokes for an IC engine cycle would be used as 2 cycles in a pump, the pumped fluid would be 2 x 3.9L in 3 revs. The fluid pumped, configured as a true pump, in one rev would be 2.6L. I believe these are facts, based on engineering interpretation.
Read the title of the thread, it's all about displacemnt ratings, sorry you took a wrong turn. And I saw nothing related to my hypothetical 3.9L 6 cyl piston engine.
Huh?
First, the "wiki" definition is consistent with the "swept volume" defination for piston engines, which was based on the historic "piston Pump" displacement rating.
Also, you are consitently using the air pumped by an IC engine in your explanations, which would put the 13B at 3.9L, if number of engine cycles doesn't matter ( ref 2-stroke, 4-stroke).
On the subject of displacement, I suggested using a simple piston pump analogy (predated the current concept of swept volume), which puts the 2-cycle and 4 cycle at same disp't, as it is now. It also puts the 13B at 2.6L,. All are considered to be true fluid pumps, 100% VE, and displacement is rated a 1 rev.
Kevin
RE: Standard Definition of Displacement. Is there one?
- Steve
RE: Standard Definition of Displacement. Is there one?
RE: Standard Definition of Displacement. Is there one?
I've had enough.
Kevin now has 2 threads full of garbage.
Regards
Pat
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RE: Standard Definition of Displacement. Is there one?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Standard Definition of Displacement. Is there one?
Hopefully this will all disappear at least from here tonight Aus time or tomorrow morning USA time.
Oh. The other one was not a thread, but simply a post in the hand built engines thread. Unfortunately I answered before I saw just how stupid it was getting here.
Regards
Pat
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RE: Standard Definition of Displacement. Is there one?
You were looking in the wrong place for a definition.
A good old dictionary would have been just fine.
Simply put, the removal of something and then something else takes its place. Thus a piston is removed and the volume that is left is what displaced it. And since that is all pretty much set by the Bore and the stroke, its a fairly simple thing to calculate it. Oh and there is another word that is used along with the word displacement, swept volume.
And another thing, the number of cycles and or rpm has nothing to do with the displacement calculation. If you enter that factor then you are concerning yourself with CFM or capacity.
RE: Standard Definition of Displacement. Is there one?